Uplink multi-user access may be performed through different multiple access techniques such as Time Division Multiple Access, TDMA, Frequency Division Multiple Access, FDMA, Code Division Multiple Access, CDMA, and Space Division Multiple Access, SDMA. Herein, the multi-user code division multiple access communication technique is a very important category of uplink multi-user access communication techniques, which can provide excellent access performance and thus has been adopted by multiple wireless communication standards.
In the access process using code division multiple access, first, each access terminal uses a spreading sequence with a certain length (for example, a spreading sequence with a length L means that this spreading sequence consists of L symbols or L elements, and here, L symbols/L elements may be L digital symbols) to perform spreading on digital amplitude-modulated and phase-modulated data symbols. The spreading process refers to the process in which each modulated data symbol is multiplied by each symbol of the spreading sequence, and finally a symbol sequence having the same length as that of the employed spreading sequence is formed. In the spreading process, each modulated data symbol (for example, a constellation point symbol modulated by means of QAM) is multiplied by each symbol of the spreading sequence, and finally each modulated data symbol will be spread into a symbol sequence having the same length as that of the employed spreading sequence. For example, if a spreading sequence of a length L is used, each modulated symbol will be spread into L symbols, in other words, each modulated data symbol is carried on a spreading sequence with the length L. Then, the spread symbol sequences of all access terminals may be sent on the same time-frequency resources. Finally, a base station receives a combined signal on which the spreading signals of all access terminals are superimposed, and available information of each terminal is separated from the combined signal through the multi-user receiver technique.
The communication technique that applies the code division multiple access is generally categorized as spread spectrum communication because a modulation symbol of a terminal will be spread into L-times symbols. If the transmission time of the L-times symbols after spreading is required to be equal to that of the modulation symbols before spreading, the required bandwidth will be necessarily spread L times. This is also why the spreading sequence is generally referred to as the spread spectrum sequence.
If the spread symbol of each terminal is transferred through the multi-carrier technique (such as Orthogonal Frequency Division Multiplexing, OFDM, and Frequency Bank Multi Carrier, FBMC), a combination of the two techniques is referred to as the Multi-Carrier Code Division Multiple Access, MC-CDMA, technique.
In the code division multiple access technique, the spreading process at a transmitting side is relatively simple. It is only required that each modulation symbol such as each QAM-modulated symbol is multiplied by each symbol of a spreading sequence having a length of L such that L spread symbols can be obtained, and then the spread symbols can be transmitted by means of the single-carrier or multi-carrier technique. On the contrary, the receiving process of the base station is not simple.
How to realize a good code division multiple access performance, or more directly, how the base station accurately separate available data information of each terminal from the combined signal is the key of a code division multiple access system. Herein, two main aspects are involved: a spreading sequence and a receiver. Selection of the spreading sequence is a performance basis and design of the receiver is a performance guarantee.
Specifically, to realize a good access performance, good cross-correlation properties are firstly needed for spreading sequences employed by different terminals. If the spreading sequences are directly transferred in a wireless multipath channel, such as a single-carrier code division multiplexing technique, good auto-correlation properties of the sequences are required for combating time-delay multipath spreading of the sequences themselves.
Since the multi-carrier code division multiplexing technique can counter the multipath based on the multi-carrier technique, only the cross-correlation properties that facilitate user information separation needs to be considered for the spreading sequences. This is the most significant difference in the sequence selection between the single-carrier code division multiplexing technique and the multi-carrier code division multiplexing technique.
A good spreading sequence is the basis of the performance, and finally separation of multi-user information is accomplished at a base station side. Corresponding performances will be obtained when the base station employs different multi-user receiving techniques. However, if an optimum multi-user data separation performance is required to be obtained, the base station needs to employ a multi-user receiver technique with high performance and high complexity, such as the serial interference cancellation receiver technique.
Due to the importance of the spreading sequence, the main difference between different code division multiple access techniques lies in the selection of the spreading sequence. The Direct Sequence-Code Division Multiple Access, DS-CDMA, technique is the most common code division multiple access technique, and has been adopted by multiple wireless communication standards as the uplink multiple user access technique, and the spreading sequence thereof is based on the simplest binary Pseudo-Noise, PN, real number sequence. Due to the simplicity of the sequence, DS-CDMA based on the PN sequence is also one of the most common techniques for multi-carrier code division multiplexing. In this technique, each modulated symbol will be firstly spread by a binary pseudo-noise real number sequence and then transmitted by means of the multi-carrier technique.
The binary pseudo-noise real number sequence may also be referred to as a binary-system pseudo-noise sequence in which a value of each symbol is generally represented as 0 or 1, and can also be further represented as a bipolar sequence, that is, 0 is represented as +1 and 1 is represented as −1, or 0 is represented as −1 and 1 is represented as +1.
The length of the spreading sequence is also a key factor of the code division multiple access technique. The longer the spreading sequence is, the easier the low cross-correlation between the spreading sequences employed by various terminals is guaranteed, and the more sequences of low cross-correlation are easily found, and therefore simultaneous access of more terminals is supported. If the number of the simultaneously accessed terminals is larger than the length of the spreading sequence, it can be considered that the multiple user access system is in the overload state. It should be noted that the code division multiple access technique capable of achieving system overload is one of the key attributes shining in the future wireless communications.
To provide a flexible system design and support simultaneous access of more users, generally spreading sequences employed in accessing terminals are not mutually orthogonal, and in terms of multi-user information, non-orthogonal multiple access employed in the uplink can obtain larger system capacity or edge throughput than orthogonal multiple access. Since the spreading sequences of various terminals are not mutually orthogonal, and thus generally the demodulation performance of each user will be degraded as the number of simultaneously accessed users increases. The multi-user interference will become more serious when the system is overloaded. For simple realization, most of the current mainstream code division multiple access techniques use the binary pseudo-noise real number sequence as the spreading sequence. However, since the low cross-correlation between the binary pseudo-noise real number sequences, especially the binary pseudo-noise real number sequences with shorter lengths cannot be easily guaranteed, a serious multi-user interference will be caused and performance of multi-user access will be inevitably influenced.